In addition to changing pitch of the propeller, the invention provides several safety features. One of them relates to a brake which stops a rotating gear within a gear train which changes pitch. Stopping the gear train drives the propeller blades to a feathered condition. In order to explain how such a brake can cause feathering, it is first necessary to explain the operation of the gear train. This explanation occupies the remainder of the Background.
FIG. 1 shows an aircraft 3 powered by engines with which the invention can be used. The engines each drive a pair of counterrotating propellers. One propeller includes blades 6 and the other includes blades 9. "Counterrotating" means that the blades rotate in opposite directions about a common axis 67, as indicated by arrows 12 and 15.
The blades are of the variable-pitch type, meaning that blades 6 and 9 can rotate about respective pitch axes 6A and 9A (shown in FIG. 2), as indicated by arrows 33 and 36. Changing pitch allows the angle-of-attack of the blades to be optimized for the prevailing engine power level and flight conditions.
FIG. 2 shows a type of turbine system which can drive the counterrotating propellers. A gas generator (not shown) provides a hot gas stream 30. The gas stream impinges upon two counterrotating turbines 18 and 24. Turbine 24 is supported by a stationary frame 37 by bearings 27A and 27B. Turbine 18 rides upon turbine 24, by means of bearings 21A and 21B. The propeller blades 6 and 9 are directly connected to the turbines 18 and 24, and rotate at the same speeds as the respective turbines.
Shafts 49 and 87 are connected to blades 6 and 9, and a respective bevel gear 45 or 90 is connected to each shaft. Rotation of the shafts causes pitch of the blades to change. A system which drives the shafts is shown in FIG. 3.
FIG. 3 shows, in simplified, exemplary, schematic form, one type of gear train which can accomplish the change in pitch. A pair of bevel ring gears 51 and 53, which are concentric about the engine axis 67, both simultaneously engage the blade bevel gear 45. When the bevel ring gears rotate in opposite directions, they rotate the blade bevel gear 45, causing pitch to change. The bevel ring gears are each affixed to a respective ring gear 56 or 58. A compound planet gear, comprising sub-planets 61A and 61B affixed to each other, engages the ring gears 56 and 58, but at different gear ratios. That is, the ratio between sub-planet 61A and ring gear 56 is different from the ratio between sub-planet 61B and ring gear 58. Because of the different gear ratios, when the planet gear rotates, the ring gears 56 and 58 rotate in opposite directions, causing pitch to change.
The planet gear is driven by a sun gear 63, which is, in turn, driven by a ring gear 66 attached to it by means of shell 68. A motor 72 drives the latter ring gear 66 by means of a pinion gear 75, causing the change in pitch. A similar gear train drives the other blade bevel gear 90.
It is significant that, with the system of FIG. 3, when pitch is unchanging, the sun gear 63 is required to rotate at synchronous speed with the propeller blade 6. Consequently, the pinion gear 75 must rotate constantly. Further, if there is no disengagement mechanism provided between the motor 72 and the pinion gear 75, the motor 72 also must be constantly rotating. Such constant rotation can be disadvantageous in some situations. FIG. 4 illustrates a more detailed gear train which eliminates the requirement of constant motor rotation.
In a general sense, FIG. 4 adds a differential 130 which subtracts the speed of the propeller from the speed of the sun gear 63 in FIG. 3, making the motor 72 stationary when pitch is not changed. When pitch change is desired, the pinion 112 is rotated relatively to the pinion 115 which is always rotating at a speed proportional to the propeller speed (which is driven by ring gear 109).
In FIG. 4, some additional components to those in FIG. 3 are shown. For example, shaft 49 does not connect to the blade 6 directly, but, instead, connects through a planetary torque multiplier 52. The torque multiplier allows the shaft 49 to carry less torque, and thus to be manufactured at a smaller diameter. The reduction in diameter is advantageous because the shaft 49 passes through a turbine blade 46 (see FIG. 2 or 3). With the reduced shaft diameter, the aerodynamic designers of the turbine blade 46 need not significantly redesign the blade 46 in order to contain the shaft 49.
A second difference between FIGS. 3 and 4 is that there are three ring gears (i.e., 56A, 58A, and 59) engaging three sub-planets (i.e., 95, 96, and 97), instead of two ring gears with two sub-planets as in FIG. 3. Two of the three ring gears in FIG. 4, labeled 56A and 58A, can be termed movable ring gears, because they move with respect to the blade 6 when pitch changes. (A third movable ring gear will be introduced later.) The remaining ring gear, labeled 59, can be termed a fixed ring gear because it is fastened to the blade 6 by a frame 100, and remains synchronous with the blade 6 at all times. (A second fixed ring gear is introduced later.)
All three sub-planets 95, 96, and 97 are locked together on a common shaft. The three gear ratios between the sub-planets and their respective ring gears (i.e. sub-planet 95/ring gear 56A, sub-planet 96/ring gear 58A, and sub-planet 97/ring gear 59) are different. Consequently, when the proper ratios are used, then as the sub-planets rotate about their axis 102, they cause movable ring gears 56A and 58A to rotate in opposite directions relative to each other. Further, the movable ring gears mole with respect to the fixed ring gear 59: the movable ring gears view the fixed ring gear as stationary.
It is noted that, when pitch is unchanging, the two movable ring gears 56A and 58A rotate at synchronous speed with blade 6 and the fixed ring gear 59. There is no relative rotation between any of them. Further, under these conditions, the sub-planets are not rotating about their own axis 102, but are orbiting about the engine centerline 67.
To cause a change in pitch, a sun gear 101 rotates with respect to the fixed ring gear 59. This sun gear is driven by a third movable ring gear 107, by means of shell 104. Two pinion gears 112 and 115, mounted on respective shafts 121 and 118, cause the change in pitch. (Pinions 112 and 115 do not orbit about the centerline 67; they are fixed, as are all components located to the right of pinion 115.)
For the pitch change to occur, relative rotation is required between the two pinions 112 and 115, which causes third movable ring gear 107 to move with respect to the second fixed ring gear 109. Consequently, the sun gear 101 moves with respect to the first fixed ring gear 59, causing the sub-planets to rotate about their axis 102, and to change their orbital speed slightly, because of the rotation about their axis 102. Rotation of the sub-planets causes the first and second movable ring gears 56A and 58A to rotate in opposite relative directions, rotating bevel pinion 45 and thus changing pitch.
As thus far described, the pinion gears 112 and 115 in FIG. 4 experience constant rotation. However, this constant rotation is decoupled from the motor 72 by a differential 130, which will now be described.
A key feature of the differential is a planetary system comprising sun gear 144, planet gears 141 (carried by a carrier 138), and ring gear 147. The planet carrier 138 rotates constantly at a speed proportional to that of pinion 115. Similarly, the ring gear 147 rotates constantly at a speed equal to that of pinion 112. The constants of proportionality are chosen (by choosing the proper gear ratios) such that the sun gear 144 is stationary when pinions 112 and 115 rotate at equal speeds. Consequently, motor 72 (which drives sun gear 144) does not rotate when pitch change is absent.
Further, when the motor 72 does rotate, pinions 112 and 115 rotate at different speeds: pinion 112 either accelerates or decelerates with respect to pinion 115, depending on the direction of rotation of the motor 72. If pinion 112 decelerates, pitch changes in one direction, for example, toward feathered pitch. If the pinion 112 accelerates, pitch changes in the other direction, for example, toward flat (or "fine") pitch. (The terms flat and feather are explained with reference to FIGS. 10 and 11, which are discussed later.)
The present invention is concerned with part of a control system which drives the motor 72, when the motor is of the hydraulic type. The invention also includes a brake which stops shaft 121 in FIG. 4, which drives the propeller toward feathered pitch, irrespective of the action of the motor 72.